Web app allows you to:
1) duplicate your own playlists
2) clone the videos of someone else's playlist into your own account
3) merge someone else's playlist with any of of your existing playlist.
You can also set whether the duplicated playlist will be Public, Private or Unlisted.
How it works: Sign-in with your YouTube account and then enter the URL of any YouTube Playlist that you wish to clone.
My comment: Another little jewel by Amit Agarwal of Digital Inspiration. This is a precious tool for video curators who need to copy, duplicate or even merge together existing youtube playlists for different purposes.
Try it out now: https://ctrlq.org/youtube/playlists/